1. Field of the Invention
The invention relates generally to data compression methods and in particular to an improvement of the JPEG method for image compression as it is applied to seismic data.
2. Description of the prior art
The JPEG standard algorithm for image compression (Pennebaker and Mitchell, 1993) consists of the following three steps, performed for each 8.times.8 block of pixels in a two-dimensional array:
1. Transform the 8.times.8 block of pixels, using a discrete cosine transform. PA1 2. Quantize (scale and round) the transform coefficients into small integers. PA1 3. Encode the bits, using few bits to represent the most frequent integers. The decompression algorithm inverts each of these steps, in reverse order. Both algorithms may be easily extended to compress and decompress arrays of any dimension. PA1 Bradley, J. N., C. M., and Hopper, T., 1993, The FBI wavelet/scalar quantization standard for gray-scale fingerprint image compression: Visual Information Processing II, SPIE Proceedings, 293-304. (ftp://ftp.c3.lanl.gov/pub/WSQ.) PA1 Jawerth, B., and Sweldens, W., 1995, Biorthogonal smooth local trigonometric bases: J. Fourier Anal. Appl., 2. (http:H/cm.bell-labs.com/who/wim/papers/-papers.htnl). PA1 Jawerth, B., Liu, Y., and Sweldens, W., 1996, Signal compression with smooth local trigonometric bases: http:flcm.bell-labs.com/who/wim/papers/-papers.html. PA1 Malvar, H. S., and Staelin, D. H., 1989, The LOT - transform coding without blocking effects: IEEE Transactions on Acoustic, Speech, and Signal Processing, 37, no. 4, 553-559. PA1 Malvar, H. S., 1990, Lapped transforms for efficient transform/subband coding: IEEE Transactions on Acoustic, Speech, and Signal Processing, 38,no. 6, 969-978. PA1 Pennebaker, W. B., and Mitchell, J. L., 1993, JPEG still image data compression standard: Van Nostrand Reinhold. PA1 Princen, J. P., and Bradley, A. B., 1956, Analysis/synthesis filter bank design based on time domain aliasing cancellation: IEEE Transaction on Acoustics, Speech, and Signal Processing, 34, no. 5, 1153-1161. PA1 Wickerhauser, M. V., 1994, Adapted wavelet analysis from theory to software: A. K. Peters. PA1 Yeo, B., and Liu, B., 1995, Volume rendering of DCT-based compressed 3D scalar data: IEEE Transactions on Visualization and Computer Graphics, 1,no. 1, 29-43.
FIG. 1 displays a 2-D array of seismic data that has not been compressed. The 2-D array of 32-bit floating-point numbers of FIG. 1 is a constant-time slice extracted from a 3-D seismic survey. FIG. 2 displays a zoomed subset of the same array.
FIG. 3 shows the same zoomed subset, after compression and decompression of the entire 3-D array using a JPEG-like algorithm. The compression ratio for the entire array is approximately 103:1, meaning that the original array of 32-bit floating-point numbers contained about 103 times as many bits as the compressed array.
For such large compression ratios, this JPEG-like algorithm produces the blocking artifacts visible in FIG. 3. At lower compression ratios, these discontinuities between blocks become less visible, but they may still be significant, particularly when further processing or interpretation is performed after decompression.
The artifacts in FIG. 3 are the result of each block of 8.times.8 samples being compressed and decompressed, independently, with no attempt to maintain continuity between the blocks.
In spite of these artifacts, the ability to compress and decompress such small subsets of data independently is a desirable feature. In particular, it enables access to some small part of a large compressed array, without decompressing the entire array. It also enables the compression algorithm to adapt to spatial variations in data amplitude and spectrum. These features are lacking in compression methods based on wavelet transforms (e.g., Bradley et al., 1993; Wickerhauser, 1994). The problem addressed in this application is to obtain these features, without the blocking artifacts.
One solution to this problem is to compress data using overlapping blocks, so that decompressed sample values can be computed without reference to values adjacent to block boundaries. This solution was used by Yeo and Liu (1995), in their adaptation of the JPEG algorithm to volume rendering of 3-D medical images. Unfortunately, the use of overlapping blocks increases the number of blocks to be compressed, which increases computation times and decreases compression ratios.
3. References to Prior Work Cited in this Specification
4. Identification of Objects of the Invention
It is a primary object of the invention to provide an improved JPEG algorithm for data compression.
An important object of the invention is to provide an improved JPEG data compression algorithm which can be used to compress small subsets of data without blocking artifacts.